1. Field of the Invention
The present invention relates generally to memory devices and, more particularly, to charge pump circuits for use an electrically reprogrammable non-volatile semiconductor memory devices.
2. Description of Related Art
A typical electrically reprogrammable non-volatile semiconductor memory device is a electrically erasable programmable read-only memory (EEPROM). U.S. Pat. No. 5,394,372, the entire contents of which are incorporated herein by reference to the extent compatible with the present invention, discloses a charge-pump system for use with such a memory, wherein the frequency is attenuated with increases in VDD. In this circuit architecture, however, the frequency is also reduced with increases in temperature. U.S. Pat. No. 6,064,275, the contents of which are incorporated herein by reference to the extent compatible with the present invention, discloses MOSFET having resistances that increases with temperature increases so that current produced by the circuit""s current mirror is not optimally controlled.
In accordance with the present invention, a current mirror of a clock generator increases the reference current with increases in temperature, so that the charging time of the capacitors is increased to thereby increase a frequency of the clock generator with increases in temperature. Accordingly, the clock generator of the present invention, which is preferably used in combination with a charge pump, decreases in frequency with increases in VDD and increases in frequency with increases in temperature.
In accordance with another aspect of the present invention, the resistance is constant so that the current produced by the current mirror can be controlled. The resistance can be independent of temperature.
In one implementation, the current produced by the current mirror is controlled by a temperature dependent, current-adjusting MOSFET which has a threshold voltage (Vt) that varies with temperature. As the temperature varies, the current through the a temperature dependent, current-adjusting MOSFET varies. The MOSFET can be provided with a temperature-independent power supply, so that the current of the temperature dependent, current-adjusting MOSFET can be more closely controlled.
The various embodiments of the present invention may include or address one or more of the following objectives. One objective is to provide a clock generator whose frequency reduces as VDD increases and increases as the temperature increases. Using the clock generator in connection with a charge pump system can stabilize pump current and improve differential of Vclamp between high VDD and low VDD. Use of the charge pump system on a flash memory can improve the programming performance in low VDD and high temperature. Moreover, the charge pump system can reduce power consumption when programming in high VDD.
Any feature or combination of features described herein are included within the scope of the present invention provided that the features included in any such combination are not mutually inconsistent as will be apparent from the context, this specification, and the knowledge of one of ordinary skill in the art. Additional advantages and aspects of the present invention are apparent in the following detailed description and claims.